GB2037270A

GB2037270A - Screen-printing ink containing a reducible metal oxide

Info

Publication number: GB2037270A
Application number: GB7930697A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1978-09-08
Filing date: 1979-09-05
Publication date: 1980-07-09
Also published as: NL7906601A; FR2437427A1; JPS6210552B2; DE2935421C2; DE2935421A1; FR2437427B1; GB2037270B; JPS5538882A

Abstract

A screen-printing ink suitable for making thick film circuits and which comprises (i) an "active material", (ii) a vitreous material, and (iii) an organic binder, is characterised by the inclusion of (iv) 1-10 vol.% of at least one metal oxide which, during firing in a neutral or weakly-reducing atmosphere, oxidatively eliminates the organic binder while itself being reduced to a lower valency oxide which is incorporated in the vitreous phase. Exemplified constituents are (i) micronised conductive metals e.g. Cu, Ni; metal hexaborides; dielectric compounds e.g. Al2O3; (ii) a fritted zinc borosilicate or calcium boroaluminate glass; (iii) ethyl cellulose in terpineol; (iv) a cerium oxide or praseodymium oxide. The ink may be printed onto alumina and fired in N2 or N2 + H2.

Description

SPECIFICATION Screen-printing ink which is fired in a non-oxidizing atmosphere The invention relates to a screen-printing ink fired in a neutral or weakly reducing atmosphere, comprising an active material, a passive, vitreous material and an organic binder, this binder being completely removed during firing. The invention also relates to electric components which comprise at least one layer obtained by screen-printing with this ink and firing in a neutral or a weakly reducing atmosphere.

Such a screen-printing ink is used in the field of microelectronics, particularly for making thick film components by screen-printing on substrates generally consisting of aluminium oxide, such as hybrid circuits (converters, filters, networks having conductors, resistors and capacitors).

Such screen-printing inks find frequent use in industry, but the use of noble metals, such as gold, silver or palladium, as the metal active material renders these products relatively expensive. During the course of investigations into screen-printing inks, a range of inks containing no noble metals and which must be fired in a nitrogen atmosphere or in a weakly reducing atmosphere has been developed. Thus, United Kingdom Patent Specification 1,489,031 describes a screen-printing ink based on copper, West German Offenlegungsschrift 2,755,935 describes a compatible insulating screen-printing ink and our co-pending Patent Application 7,915,805 describes a compatible resistor obtained by screen-printing.

These screen-printing inks are prepared by mixing glass powder with conducting particles or with dielectric ceramic particles, and suspending the mixture in an organic binder. A pattern of the ink produced on a substrate by screen-printing, is fired in a neutral or weakly reducing atmosphere. Owing to the nature of the firing atmosphere, the organic binder may not be completely removed from the screen-printed layer and that contaminations remain behind which affect the intrinsic properties of said layers adversely. An object of the invention is to mitigate this drawback.

The invention provides a screen-printing ink which is fired in a neutral or weakly reducing atmosphere (as hereinafter defined) and comprises an active material, a passive vitreous material and an organic binder, this binder being completely removed during firing, characterized in that the ink contains from 1 to 10% by volume of one or more metal oxides which can be reduced to a lower valency metal oxide during firing of the ink. Throughout this Specification, a neutral or weakly reducing atmosphere is understood to be a neutral or weakly reducing atmosphere containing not more than 10 p.p.m. of oxygen. Such an atmosphere may consist, for example of pure nitrogen or of a mixture of nitrogen and hydrogen containing 10% by volume of hydrogen.

In this manner these metal oxides act as oxidizing agents with respect to the organic binder, which can therefore be removed during firing without carbonization. The metal oxides are reduced to a lower valency and are incorporated in the vitreous phase without significantly changing the electric properties thereof. This particularly holds for insulating materials as regards the dielectric losses and the insulation resistance.

It should be noted that United States Patent Specification 3,609,483 describes a screen-printing paste comprising cerium oxide which is used for thick film capacitors. However, the cerium oxide in this paste is used as a substituent of the barium titanate in the dielectric basic compound and is not dispersed in the organic binder as an oxidizing agent for this binder. This is also the case for the ceramic material described in United States Patent Specification 3,682,766, which comprises praseodymium oxide as the basic element of the ceramic.

United Kingdom Patent Specification 1,349,163 describes a protecting glass to which an oxide of a transition element is afterwards added to reduce the mobility of the alkali metal ions which might spoil the semiconductor element to be produced. Since these oxides are fired in an oxidizing atmosphere, these oxides are, however, reduced to different degress of oxidation, which cause electronic conduction in the glass owing to their simultaneous presence. This is detrimental to the components which are the subject of the present invention.

In one aspect of the invention, these oxides are selected from the cerium and the praseodymium oxides which can be reduced to lower valency metal oxides during firing of the ink. Preferably, praseodymium oxide Pr6O11 and/or PrO2 are added to the organic binder in a quantity by volume between 1 and 10%, as this is more easily dissolved in the glass.

An embodiment of the invention will now be described with reference to the following Example.

Example In accordance with the conventional methods for the preparation of screen-printing inks, the active material, for example a conductive metal such as Cu or Ni, a metal compound, for example MeB6 or a dielectric compound such as Awl203, are ground into powder having an average particle size of the order of a micron. The passive material, for example a glass consisting of metal oxides and capable of being fired in a neutral or weakly reducing atmosphere, isfritted.

Suitable glasses, which are described in the above-mentioned Patent Applications, are zinc-borosilicate or calcium-boroaluminate glasses. The organic binder consists of a solution of a resin, for example, ethyl cellulose in terpineol, care being taken that the resin has completely dissolved and that the solution has a desired viscosity. The binder, the active material and the passive material are mixed in a mixing device with high shearing rates to obtain the screen-printing ink.

According to the present invention this ink, in quantities up to 10% by volume, one or more reducible metal oxides which can be added to any stage in the preparation of the ink. Preferably, this oxide is an oxide of a metal from group Ill-A of the periodic system of elements, such as cerium or praseodymium, which is first dispersed in the binder.

In accordance with one embodiment for obtaining a dielectric screen-printing paste as described in the above-mentioned German Offenlegungsschrift 2,755,935,45% by volume of pulverulent aluminium oxide is mixed with 55% by volume of glass powder having the following composition (expressed in molar percent): ZnO 20% SiO2 40% B203 15% A1203 5% Cao 5% SrO 15% Thereafter this mixture is dispersed in an organic binder, for example a solution of ethyl cellulose in terpineol. After screen-printing the desired pattern of the ink on a substrate, the assembly is fired in nitrogen having a residual oxygen content of 10 p.p.m. The layers thus obtained have a certain friability and porosity owing to poor wetting of the glass by organic contaminations. When the ink, contains 2% by volume of Pr2O11, the cohesion and the density of the layers produced using this ink are improved in a reproducible manner. With a lower residual oxygen content than 10 p.p.m., it is sufficient to increase the quantity of oxide.

The following Table shows the electric properties of the layers obtained without and with the addition of praseodymium oxide: TABLE without addition with PreOii 1kHz 10 kHz 1kHz 10kHz 5 5 5 7 7 103tan ò 13 1 0.5 0.7 A similar effect is obtained with other screen-printing inks according to the invention which are fired in a non-oxidizing atmosphere.

Claims

1. A screen-printing inkwhich is fired in a neutral or weakly-reducing atmosphere (as hereinbefore defined) and comprises an active material, a passive, vitreous material and an organic binder, this binder being completely removed during firing, characterized in that the ink contains from 1 to 10% by volume of one or more metal oxides which can be reduced to a lower valency metal oxide during firing of the ink.

2. A screen-printing ink as claimed in Claim 1, characterized in that the metal oxide(s) is (are) a cerium oxide andior a praseodymium oxide which can be reduced to a lower valency metal oxide during firing of the ink.

3. A screen-printing ink as claimed in Claim 1 or Claim 2, characterized in that it contains between 1 and 10% by volume of Pr6O11 andior PrO2.

4. A screen-printing ink as claimed in Claim 1, substantially as herein described with reference to the Example.

5. An electric component comprising a substrate bearing a layerformed by screen-printing a screen-printing ink as claimed in any preceding Claim onto the substrate, drying the ink and firing the dried ink in a neutral or weakly-reducing atmosphere (as hereinbefore defined).